Andreas Hindborg <a.hindborg@xxxxxxxxxx> writes: > "Gary Guo" <gary@xxxxxxxxxxx> writes: > >> On Sat, 5 Oct 2024 13:59:44 +0200 >> Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote: >> >>> On Sat, Oct 5, 2024 at 11:49 AM Andreas Hindborg <a.hindborg@xxxxxxxxxx> wrote: >>> > >>> > Hi Greg, >>> > >>> > "Greg KH" <gregkh@xxxxxxxxxxxxxxxxxxx> writes: >>> > >>> > > On Fri, Oct 04, 2024 at 04:52:24PM +0100, Gary Guo wrote: >>> > >> There is an operation needed by `block::mq`, atomically decreasing >>> > >> refcount from 2 to 0, which is not available through refcount.h, so >>> > >> I exposed `Refcount::as_atomic` which allows accessing the refcount >>> > >> directly. >>> > > >>> > > That's scary, and of course feels wrong on many levels, but: >>> > > >>> > > >>> > >> @@ -91,13 +95,17 @@ pub(crate) unsafe fn start_unchecked(this: &ARef<Self>) { >>> > >> /// C `struct request`. If the operation fails, `this` is returned in the >>> > >> /// `Err` variant. >>> > >> fn try_set_end(this: ARef<Self>) -> Result<*mut bindings::request, ARef<Self>> { >>> > >> - // We can race with `TagSet::tag_to_rq` >>> > >> - if let Err(_old) = this.wrapper_ref().refcount().compare_exchange( >>> > >> - 2, >>> > >> - 0, >>> > >> - Ordering::Relaxed, >>> > >> - Ordering::Relaxed, >>> > >> - ) { >>> > >> + // To hand back the ownership, we need the current refcount to be 2. >>> > >> + // Since we can race with `TagSet::tag_to_rq`, this needs to atomically reduce >>> > >> + // refcount to 0. `Refcount` does not provide a way to do this, so use the underlying >>> > >> + // atomics directly. >>> > >> + if this >>> > >> + .wrapper_ref() >>> > >> + .refcount() >>> > >> + .as_atomic() >>> > >> + .compare_exchange(2, 0, Ordering::Relaxed, Ordering::Relaxed) >>> > >> + .is_err() >>> > > >>> > > Why not just call rust_helper_refcount_set()? Or is the issue that you >>> > > think you might not be 2 here? And if you HAVE to be 2, why that magic >>> > > value (i.e. why not just always be 1 and rely on normal >>> > > increment/decrement?) >>> > > >>> > > I know some refcounts are odd in the kernel, but I don't see where the >>> > > block layer is caring about 2 as a refcount anywhere, what am I missing? >>> > >>> > It is in the documentation, rendered version available here [1]. Let me >>> > know if it is still unclear, then I guess we need to update the docs. >>> > >>> > Also, my session from Recipes has a little bit of discussion regarding >>> > this refcount and it's use [2]. >>> > >>> > Best regards, >>> > Andreas >>> > >>> > >>> > [1] https://rust.docs.kernel.org/kernel/block/mq/struct.Request.html#implementation-details >>> > [2] https://youtu.be/1LEvgkhU-t4?si=B1XnJhzCCNnUtRsI&t=1685 >>> >>> So it sounds like there is one refcount from the C side, and some >>> number of references from the Rust side. The function checks whether >>> there's only one Rust reference left, and if so, takes ownership of >>> the value, correct? >>> >>> In that case, the CAS should have an acquire ordering to synchronize >>> with dropping the refcount 3->2 on another thread. Otherwise, you >>> might have a data race with the operations that happened just before >>> the 3->2 refcount drop. >>> >>> Alice >> >> The code as is is fine since there's no data protected in >> `RequestDataWrapper` yet (in fact it's not even generic yet). I know >> Andreas does want to introduce driver-specific data into that, so in >> the long term the acquire would be necessary. >> >> Andreas, please let me know if you want me to make the change now, or >> you'd rather change the ordering when you introduce data to >> `RequestDataWrapper`. > > I guess we will have said data dependencies when we are going to run > drop for fields in the private data area. Thanks for pointing that out. > I will update the ordering when I submit that patch. > > As I mentioned before, I would rather we do not apply this patch before > we get a way to inline helpers. As discussed offline, the code that suffers the performance regression is downstream, and since this change seems to be important, I can apply the helper LTO patch downstream as well. Since the plan for the downstream code _is_ to move upstream, I really hope to see the helper LTO patch upstream, so we don't get a performance regression because of these refcounts. If we cannot figure out a way to get the LTO patches (or an alternative solution) upstream, we can always revert back to a more performant solution in block. BR Andreas